asctime.3p (5910B)
- '\" et
- .TH ASCTIME "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
- .\"
- .SH PROLOG
- This manual page is part of the POSIX Programmer's Manual.
- The Linux implementation of this interface may differ (consult
- the corresponding Linux manual page for details of Linux behavior),
- or the interface may not be implemented on Linux.
- .\"
- .SH NAME
- asctime,
- asctime_r
- \(em convert date and time to a string
- .SH SYNOPSIS
- .LP
- .nf
- #include <time.h>
- .P
- char *asctime(const struct tm *\fItimeptr\fP);
- char *asctime_r(const struct tm *restrict \fItm\fP, char *restrict \fIbuf\fP);
- .fi
- .SH DESCRIPTION
- For
- \fIasctime\fR():
- The functionality described on this reference page is aligned with the
- ISO\ C standard. Any conflict between the requirements described here and the
- ISO\ C standard is unintentional. This volume of POSIX.1\(hy2017 defers to the ISO\ C standard.
- .P
- The
- \fIasctime\fR()
- function shall convert the broken-down time in the structure pointed
- to by
- .IR timeptr
- into a string in the form:
- .sp
- .RS 4
- .nf
- Sun Sep 16 01:03:52 1973\en\e0
- .fi
- .P
- .RE
- .P
- using the equivalent of the following algorithm:
- .sp
- .RS 4
- .nf
- char *asctime(const struct tm *timeptr)
- {
- static char wday_name[7][3] = {
- "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
- };
- static char mon_name[12][3] = {
- "Jan", "Feb", "Mar", "Apr", "May", "Jun",
- "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
- };
- static char result[26];
- .P
- sprintf(result, "%.3s %.3s%3d %.2d:%.2d:%.2d %d\en",
- wday_name[timeptr->tm_wday],
- mon_name[timeptr->tm_mon],
- timeptr->tm_mday, timeptr->tm_hour,
- timeptr->tm_min, timeptr->tm_sec,
- 1900 + timeptr->tm_year);
- return result;
- }
- .fi
- .P
- .RE
- .P
- However, the behavior is undefined if \fItimeptr\fR\->\fItm_wday\fR or
- \fItimeptr\fR\->\fItm_mon\fR are not within the normal ranges as
- defined in
- .IR <time.h> ,
- or if \fItimeptr\fR\->\fItm_year\fR exceeds
- {INT_MAX}\-1990,
- or if the above algorithm would attempt to generate more than 26 bytes
- of output (including the terminating null).
- .P
- The
- .BR tm
- structure is defined in the
- .IR <time.h>
- header.
- .P
- The
- \fIasctime\fR(),
- \fIctime\fR(),
- \fIgmtime\fR(),
- and
- \fIlocaltime\fR()
- functions shall return values in one of two static objects: a
- broken-down time structure and an array of type
- .BR char .
- Execution of any of the functions may overwrite the information
- returned in either of these objects by any of the other functions.
- .P
- The
- \fIasctime\fR()
- function need not be thread-safe.
- .P
- The
- \fIasctime_r\fR()
- function shall convert the broken-down time in the structure pointed to
- by
- .IR tm
- into a string (of the same form as that returned by
- \fIasctime\fR(),
- and with the same undefined behavior when input or output is out of
- range) that is placed in the user-supplied buffer pointed to by
- .IR buf
- (which shall contain at least 26 bytes) and then return
- .IR buf .
- .SH "RETURN VALUE"
- Upon successful completion,
- \fIasctime\fR()
- shall return a pointer to the string.
- If the function is unsuccessful, it shall return NULL.
- .P
- Upon successful completion,
- \fIasctime_r\fR()
- shall return a pointer to a character string containing the date and
- time. This string is pointed to by the argument
- .IR buf .
- If the function is unsuccessful, it shall return NULL.
- .SH ERRORS
- No errors are defined.
- .LP
- .IR "The following sections are informative."
- .SH EXAMPLES
- None.
- .SH "APPLICATION USAGE"
- These functions are included only for compatibility with older
- implementations. They have undefined behavior if the resulting string
- would be too long, so the use of these functions should be
- discouraged. On implementations that do not detect output string
- length overflow, it is possible to overflow the output buffers in such
- a way as to cause applications to fail, or possible system security
- violations. Also, these functions do not support localized date and
- time formats. To avoid these problems, applications should use
- \fIstrftime\fR()
- to generate strings from broken-down times.
- .P
- Values for the broken-down time structure can be obtained by calling
- \fIgmtime\fR()
- or
- \fIlocaltime\fR().
- .P
- The
- \fIasctime_r\fR()
- function is thread-safe and shall return values in a user-supplied
- buffer instead of possibly using a static data area that may be
- overwritten by each call.
- .SH RATIONALE
- The standard developers decided to mark the
- \fIasctime\fR()
- and
- \fIasctime_r\fR()
- functions obsolescent even though
- \fIasctime\fR()
- is in the ISO\ C standard due to the possibility of buffer overflow. The ISO\ C standard
- also provides the
- \fIstrftime\fR()
- function which can be used to avoid these problems.
- .SH "FUTURE DIRECTIONS"
- These functions may be removed in a future version.
- .SH "SEE ALSO"
- .IR "\fIclock\fR\^(\|)",
- .IR "\fIctime\fR\^(\|)",
- .IR "\fIdifftime\fR\^(\|)",
- .IR "\fIgmtime\fR\^(\|)",
- .IR "\fIlocaltime\fR\^(\|)",
- .IR "\fImktime\fR\^(\|)",
- .IR "\fIstrftime\fR\^(\|)",
- .IR "\fIstrptime\fR\^(\|)",
- .IR "\fItime\fR\^(\|)",
- .IR "\fIutime\fR\^(\|)"
- .P
- The Base Definitions volume of POSIX.1\(hy2017,
- .IR "\fB<time.h>\fP"
- .\"
- .SH COPYRIGHT
- Portions of this text are reprinted and reproduced in electronic form
- from IEEE Std 1003.1-2017, Standard for Information Technology
- -- Portable Operating System Interface (POSIX), The Open Group Base
- Specifications Issue 7, 2018 Edition,
- Copyright (C) 2018 by the Institute of
- Electrical and Electronics Engineers, Inc and The Open Group.
- In the event of any discrepancy between this version and the original IEEE and
- The Open Group Standard, the original IEEE and The Open Group Standard
- is the referee document. The original Standard can be obtained online at
- http://www.opengroup.org/unix/online.html .
- .PP
- Any typographical or formatting errors that appear
- in this page are most likely
- to have been introduced during the conversion of the source files to
- man page format. To report such errors, see
- https://www.kernel.org/doc/man-pages/reporting_bugs.html .